Physical simulation- how to create anti-gravity in objects like hot-air balloons
What kind of object?
Rigid body
An individual rigid body cannot be made anti-gravity, however you can make the entire scene gravity point upwards by changing Z=-9.8 to a positive number in the Scene Gravity settings:

&quot;Gravity&quot; is a property that can be keyframed, i.e. you can make it change at a given frame.
Cloth or soft body
For cloths and soft bodies you can choose what Gravity each object is feeling. There is a &quot;Field Weight&quot; panel in the Physics settings. There you can change Gravity to a negative number (assuming that you don't change the Gravity scene settings!). The slider won't allow you to do that, but you can just type something like -0.5.

Again, this can be keyframed.
Instead of keyframing
To reverse gravity at a given location rather than time, you can add a Wind force field (ShiftA &gt; Force Field &gt; Wind), make it point downwards, and changing its falloff type to Tube and falloff Power to something between 1 and 2.

Here is a scene with a Soft Body hot air balloon, with its Gravity Field Weight set to -0.3 (Scene Gravity is untouched), and with a Wind Force Field at a given altitude (also a Turbolence Force Field for more wiggling):

Here's the blend if you want to look at how it's done